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Red light

  1. Jul 30, 2004 #1
    Why the stop lights at the back of cars are red ?
    Some say because red light is visible from far distances.
    If that's true, is any relation between this property of red light and its wave length ?
  2. jcsd
  3. Jul 30, 2004 #2
    yes, because red light has a higher wavelength (750nm), this means it has the abillity to diffract more, since waves with larger wavelengths diffract more. This is a problem with the auromobile industry, the light up dials on a dashboard are usually blue, they should be red, since blue is the hardest colour to see. Do an experiment, place blue light on a clock and watch the time pass, and then do it in red light, youll see the difference clearly.
  4. Jul 30, 2004 #3
    I believe the easiest light to see is yellow ( sunlight ) but we are accustomed to certain colors meaning something as well i.e
    yellow -- warning
    red --- danger
    green -- no problem
  5. Jul 30, 2004 #4
    the eye is the most sencitive to yellow and green light, that is true, but red light gives the most detail because of its diffraction abilities.
  6. Jul 30, 2004 #5

    Doc Al

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    Staff: Mentor

    Red light would give the least detail since it diffracts more than blue light.
  7. Jul 30, 2004 #6


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    Science Advisor

    I think that RayJohn has the right idea; the reason brake lights and stoplights and stop signs are red has more to do with psychology than the physics of red light.
  8. Jul 30, 2004 #7
    Nope, I know this for a fact, it might be used for psychological reasons, but red light allows you to see in better detail tha blue light. Doc Al puts up a good point, but try it for yourself, youll see the result.
  9. Jul 30, 2004 #8


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    Some studies have indicated that yellow vehicles are the easiest for drivers to see at a distance or in bad weather, and for that reason some municipalities have yellow fire trucks. From that I might think that yellow lights would be the easiest to see at a distance or through fog.

    At any rate, I always assumed that red was chosen for stoplights because of its association with spilled blood, i.e. Danger ahead!
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